Bolometers



United States Patent 3,069,644 BOLOMETERS Edward H. Eberhardt, Fort Wayne, Ind., assignor to International Telephone and Telegraph Corporation Filed Feb. 16, 1959, Ser. No. 793,458 6 Claims. (Cl. 338-19) This is a continuation-in-part of my prior application Ser. No. 625,664, filed December 3, 1956, and entitled Bolometers," now abandoned.

This invention relates to bolometers and is particularly directed to new structures for thermally insulating the thermo-resistive element and its contacting electrodes from their physical supports.

The sensitivity of bolometers has been limited heretofore by the conduction of heat from the thermo-resistive element of its supports. Other losses of heat include radiation and conduction of heat from the element.

Also, sensitivity has been limited by the materials used in the bolometer elements per se, such materials being metals such as nickel, silver, iron, etc., or compositions having low, positive resistance temperature coefficients.

The object of this invention is to provide an improved bolometer with a view of minimizing heat loss from the thermo-resistive element and maintaining low thermal mass for rap-id response time.

Another object is to provide a bolometer in which the thermo-resistive element is formed of a material having a high, negative resistance temperature coefficient.

Still another object is to provide a method for obtaining greater sensitivities in a bolometer operating at either the same or lower values of operating power and temperatures at which the prior art bolometers are operated.

The objects of this invention are attained by forming a film or membrane of a thickness less than, for example, .00001 inch, tautened across a self-supporting frame. Onto the membrane is deposited, preferably by evaporation, a thin layer of material having a high thermal coefficient of resistance, such as lead sulphide, which forms the bolorneter element proper. Very thin films of good electrically conductive metal are evaporated on the thermo-resistive layer to make extended contact with the surface of the layer without introducing appreciable heat conductivity losses or excessive thermal mass.

This method of construction and mounting is substantially different from the usual technique wherein the bolometer element is supported 'by a bulky solid structure or by wires, ribbons or fibers and unlike such techniques, this method materially reduces all heat conduction losses to negligible values when used in conjunction with an evacuated envelope.

The low physical mass of this composite bolometer element makes the assembly non-fragile and resistance to shock and vibration, while the low thermal mass permits a rapid thermal response time without the usual resultant substantial loss of sensitivity.

The construction is such that the bolometer element pro-per need never be handled manually or with any tools at any time, except indirectly by means of the strong outer peripheral support frame. It is therefore subject to standard mass production or machine production techniques, in contrast to ordinary high sensitivity bolometers, which must be hand-made by highly skilled craftsmen.

By using such compositions as the aforementioned lead sulphide for the bolometer element, which have relatively high negative temperature coefiicients of resistance, greater sensitivities than heretofore obtainable are achieved. Also, by operating this particular element within a critical supply voltage range, disproportionately, "still higher sensitivities are realized. p 'The' above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following descrip tion of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a half-sectional view of one bolometer embodying this invention;

FIG. 2 is an enlarged detail view partly in section, and exploded, to show the films of the device of FIG. 1;

FIG. 3 is an exploded detailed sectional view of another film structure embodying this invention;

FIG. 4 is an exploded detail sectional view of yet another film structure embodying this invention;

FIG. 5 is a sectional view of another embodiment of this invention;

FIG. 6 is a sectional view taken substantially along section line 6-6 of FIG. 5;

FIG. 7 is a sectional view taken substantially along section line 7-7 of FIG. 6;

FIG. 8 is a typical circuit diagram in which the embodirnent of FIG. 6 may be used; and

FIG. 9 is a set of graphs used in explaining the operation of the invention.

In FIG. 1 is shown the envelope 1 containing the window 2 of a material having good transmissivity of the radiations to be detected. Silver chloride, or sapphire, for example, transmits with little attenuation the visible portions of the spectrum as well as the infra-red. The envelope is evacuated inasmuch as low gas pressure eliminates heat losses by convection and gas conduction.

In the example shown, the ring 3 is approximately parallel to and spaced from the window 2. Tautened across the ring is a thin membrane which may be a metallic compound. Such a membrane may be prepared by mounting a foil of aluminum, for example, on one face of the ring. One face of the foil is oxidized by anodization to a depth equal to the desired thickness of the finished membrane. Acid is applied to the other surface of the foil to selectively remove the exposed metal, leaving only the oxidized film. By suitably masking the etched side of the foil, as with a photo-resist, the central portion only of the foil need be removed, leaving an aluminum oxide membrane peripherally supported in a thin, fiat ring of aluminum foil. Tear-resistant, transparent films have been made as thin as .000002 inch thick. In FIG. 2, the reference number 5 is applied to the foil frame for supporting the aluminum oxide film 6. The foil frame is adhesively attached or clamped to the ring 3.

Additionally, a thin insulating film made in accordance with the disclosure of Lott application Ser. No. 598,976, filed July 16, 1956, now abandoned, and entitled Thin Self-Supporting Films," may be used by applying the film after it has been formed directly to the ring 3 and cementing it in place with, for example, sodium silicate cement. Thin films of silicon monoxide, collodion and Teflon (polytetrafiuroethylene) may also be used. Films of other materials may also be used so long as they have the necessary strength for supporting the various elements and electrodes and yet be thin enough to maintain thermal conductivity to a negligibly low value.

In the next step, a thin film of good electrically conductive metal is deposited on the membrane 6. The metal film 7 may be evaporated, for example, and condensed on the membrane from such metals as gold, chromium, silver, aluminum or antimony. By suitable masks, films 7 may be deposited in any desired shape covering the central portion of the membrane 6, and having a narrow conductive strip extending to one side of the frame. Electrical contact is madeat the periphery by connection .8, which leads to the lead-in pin 9.

Next, the thermo-resistive element layer is laid down on the electrode 7. Examples of suitable materials for this element are semiconductors such as antimony sulphide, lead sulphide, cadmium sulphide, nickel oxide, lead oxide, lead telluride, etc. Sulphides of many more metals are equally suitable depending upon the bolometer operating characteristics desired. Such materials are characterized by the fact that they are (1) semiconductors and (2) have relatively high, negative thermo-coefficients of resistance.

A specific thermo-resistive, semiconductor material of which the bolometer layer is composed must be chosen for suitable electrical resistance and a high thermo-resistive coefiicient. The electrical resistance depends largely on the external electrical device, such as a transistor or vacuum tube, for measuring the bolometer current or voltage, but will in genral lie between about 1,000 and 10,600,000 ohms. The thermo-resistive coefiicient should be as high as can be found, with typical values lying between 1% and 10% per degree centigrade. All of the aforementioned semiconductor materials meet these design requirements. Preferably, the layer 10 is produced by evaporating a selected semiconductor material through a suitably apertured mask. The layer it} is thin, having a typical thickness dimension of from 100 to 500 Angstroms.

Finally, the second electrode 11 is deposited over the layer it The film 11 is preferably evaporated and condensed from pure metal in the same manner as film 7. Film 11 covers the central portion of the layer 10 and includes a strip extending radially to the connector 12 and hence to the lead-in pin 13. The support film 6 must be strong to withstand the stresses caused by the films 7 and 11 as well as the thermo-resistive element 10. Because of the low thermal mass and low heat conductivity losses in the thin films described, it is possible to produce a bolometer of extremely high sensitivity without sacrificing response time.

It is possible to utilize the face-to-face resistance of the bolometer element 111} as shown in FIG. 2 or alternatively to use the edge-to-edge resistance as shown in FIG. 3. These two alternatives, as well as other possibilities, will accommodate a very wide spread in the value of the resistivity of the thermo-resistive material, and therefore a Wide choice in the selection of a suitable material. It is estimated that resistivity values from 10 to 10 ohm-cm. can be readily adapted to this bolometer, in contrast to only about 10 to 10 ohm-cm. for ordinary bolometers.

In FIG. 4 is shown another embodiment of this invention Where the ambient temperature variations will cancel out. By terminating contact film 7a short of the center of the support membrane 6 and depositing the second film 7b end-to-end with 7a and with a gap 70 between the two, the thermo-resistive layer 10 may be laid down on films 7a and 7b across the gap. A third film 7d laid down on the thermo-resistive element 10 serves as a common electrode for 7a. and 7b and has an electrical contact brought out externally in a similar fashion to electrodes to 7:1 and 7b. The external circuit can then be arranged in a bridge configuration such that common resistance variation of the thermo-resistive element due to ambient temperature variations are cancelled out.

During evaporation of the several layers, suitable masking must be provided to prevent overlap and shortcircuiting of the several metallic films.

FIGS. 5, 6 and 7 illustrate another embodiment of this invention. Like numerals indicate like parts. An evacuated glass envelope 13 has an infrared radiation transmissive window 2 and a glass base 14 which carries a series of terminal pins 15. Inside the tube is mounted the bolometer supporting structure which comprises an annular glass frame 3 to which is secured a film or membrane 16 of insulating material. As in the case of the preceding embodiments, this membrane is extremely thin but strong enough to support the various parts of the structure. The frame 3 is substantially parallel to the through companion apertures in the frame 3, these apertures being circumferentially spaced apart by approximately as shown in FIG. 6. These terminals 17, 13 and 19 are secured to the frame 3 by any suitable means so as to provide a rigid support therefor. The opposite ends of the terminal bars are rigidly connected to the terminal pins 15, respectively.

Three electrode strips 20, 21 and 22 are superposed on the insulating film 16 and are spaced apart and parallel as shown. The central strip 21 is quite narrow and extends diametrally across the frame 3, while the two outer strips 23 and 22 extend along chords of the frame immediately adjacent to the inner frame opening 23. Thus, a substantial portion of the film 16 which overlies the opening in the frame is free of other electrode structure except as will be described hereinafter. These electrodes 20, 21 and 22 are formed of one of the metals listed hereinbefore which is evaporated through a suitable mask onto the film 16, the same as the electrodes 7, 7a, 7b and 11 of the preceding embodiments.

The tips of the terminal bars 17, 18 and 15 project through the insulating film 16 as well as the strips 20, and 22, respectively, and are connected to the strips by means of a metallic paste or conductive paint such as silver paste or aquadag (conductive colloidal carbon dispersed in Water). Such a connection is indicated by the reference numeral 23a in FIG. 7.

Applied over the portion of the film 16 which coincides with a diameter of the frame opening 23 is an elongated, thin layer 1:) of thermo-resistive, semiconductor material. This layer preferably is evaporated onto the film 16 through a suitable mask and is superposed at right angles on the three strips 26, 21 and 22. The bolometer element or layer it) is therefore conductively connected to all three of these strips, one-half of the resistive portion of the layer extending between the two strips 20 and 21 and the other half extending between the two strips 21 and 22.

As shown in FIGS. 5 and 6, a semi-circular, opaque mask 24 is fixedly secured inside the envelope 13 in a position parallel to and between the window 2 and the frame 3. This mask is slightly larger in diameter than the frame 3 and covers or is in registry with one-half of the frame, as is shown more clearly in FIG. 6. By this means, the left-hand one-half of the bolometer element 10, as shown in FIG. 6, is shielded from radiation which passes through the window 2.

A typical circuit in which the bolometer of FIG. 5 may be used appears in FIG. 8. The letters R and R indicate the respective halves of the bolometer element 10, the resistor R representing the right end half of the element 10 (FIG. 6) and the resistor R representing the left-hand half which is immediately beneath the mask The bolometer is connected across a battery 25 which supplies bias voltage, with terminals 26 being connected across resistor R for measuring the current change therethrough as the bolometer is used to detect incident radiation.

Since infrared radiation passing through the window 2 can only impinge the bolometer half represented by the resistor R, only this half of the bolometer will be responsive. The remaining half of the bolometer indicated by the resistor R is never subjected to the incident radiation because of the presence of the mask 24. This half R thereby serves to compensate for any changes in ambient temperature, thereby rendering the bolometer insensitive to ambient temperature changes.

The battery 25 supplies an operating current through the bolometer element 10 (R plus R Incident radiation falling on the resistor R increases the temperature thereof. Since this resistor R is a semiconductor having a negative thermo-coefficient of resistance, this temperature increase produces a decrease in resistance. This decreased resistance results in increased current flow, producing a voltage drop across resistor R which may be'measured at the terminals 26.

:In FIG. 9 is a graph of the typical operating characteristics of a bolometer made according to the foregoing and having a bolometer element formed of the described semiconductor material. The abscissa of the graph represents relative bias power" applied to bolometer element 10, while the ordinate indicates current responsivity or sensitivity of the element. The point -1 (minus one) on the abscissa indicates the power at which the element burns out; this point constitutes an upper critical limit of power or voltage which may be applied across the element. The curve 27 represents the current change through the element for relative bias power ranging from zero to l. As will be noted, this curve 27 has two distinct portions, the first portion 28 extending from zero to X on the abscissa, and the second portion 29 extending from X to l. From zero to X, the curve tangent gradually decreases in magnitude while from X (the point of inflection) the tangent increases. This point of inflection X provides a lower critical limit having a significance which will be explained hereinafter. The term relative bias power is a function given by the following formula:

Ela G wherein B is relative power, E is the applied voltage in volts (battery 25), I is the operating current through element 10 in amperes, a is the temperature coefficient percent change in resistance per degree temperature change and G is the thermal conductance of the element 10 expressed as watts degree temperature The value of B for negative temperature coeflicient semiconductors for element 10 always has a maximum value of -1 which, as explained above, represents the value of applied power or voltage at which the element 10 burns out. The actual power or voltage at burn-out, of course, will be a finite value which differs for different semiconductors.

For values of B between zero and X, the curve portion 28 exhibits a progressively decreasing tangent value, while portion 29 indicates a progressively increasing value from the point of inflection X to the upper limit of -1. The rise in value of the tangent after the point X is a phenomenon which I have discovered to be peculiar to semiconductors having negative thermocoefiicien-ts of resistance when the circuit of FIG. 8 is used. Metals, semiconductors and still other materials having a positive thermo-coeificient of resistance cannot exhibit this increase in tangent or current responsivity in the critical range of power between the limits of X and --1. As the graph (FIG. 9) indicates, operation of the bolometer in the critical range between X and 1, results in a sharp upswing in current responsivity or sensitivity; hence, when this increased sensitivity is needed, it is only necessary to adjust the applied power to a point in this critical range.

The operating temperature of the bolometer is indicated by the curve 31, which reveals that the temperature of the bolometer increases at a fairly uniform rate until burn-out is reached. The curve 32 represents the response time of the bolometer, i.e., the time required for the bolometer to respond to a given change in incident radiation, it being evident that this response time decreases at a fairly uniform rate up to the point of burnout.

I have discovered that the use of thin, film semi- 6 conductors having negative thermal coefiicients of resistance not only provides for a materially improved sensitivity or current responsivity characteristic over the entire range of operating power, but possesses other farreaching effects such as the reduction of the size of the bias voltage supply as well as contributing to lower bolometer operating temperatures. With respect to the bias supply or battery 25, smallness results from the negative temperature coefiicient phenomenon as well as the elimination or material reduction in thermal conductivity losses in the bolometer supporting structure, which in the present instance is the insulating film indicated by the numerals 6 and 16. The normal operating temperature of the bolometer is lower than that normally encountered in bolometers of the metallic type because of the lower operating currents which result from the lower voltage bias supply. This lower operating temperature renders the bolometer more stable in operation, and, further, is reflected in the requirement of less current drain from the bias supply.

Ordinarily, with metal or semiconductive bolometers having a positive thermo-coefficient of resistance, relatively low bias voltage as well as bias power and operating temperature result in a correspondingly low bolometer current responsivity (sensitivity), such current responsivity being defined as the current change resulting from temperature change due to a change in incident radiation. However, this reduction in current responsivity is not true in the present invention, since the same values of operating voltage, power, and operating temperature are accompanied by greater current responsivity.

Thus, the present invention, which utilizes negative temperature coefiicient semiconductors in a low thermal loss configuration provides several unexpected improvements over prior art bolometers which utilize metals or semiconductors having positive temperature coefiicients. For one thing, as graphically illustrated in FIG. 9, my semiconductors provide the phenomenon of an unusually increased sensitivity for the critical range of relative bias power between X and 1 (FIG. 9), as already explained. Additionally, my bolometer requires less operating power, operates at a lower temperature, and requires a lower bias voltage supply than prior art bolometers. While these improved results inhere in the bolometer itself, suffice it to say, other improved results are realized in the type and character of equipment in which my invention may be used.

While the principles of the invention have been described in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention.

What is claimed is:

l. A bolometer comprising an annular glass frame having a central opening therethrough, a film of insulating material secured to one side of said frame to cover the opening thereof, said film having a thickness of approximately .00001 inch for obtaining a low value of thermal conductivity, a layer of semiconductor material having a negative thermo-coefficient of resistance on the central portion of said film, said layer having a thickness of approximately to 500 Angstroms, at least two terminals connected to spaced-apart portions of said layer an evacuated envelope disposed around said frame and spaced therefrom; a window in said envelope formed of a material transmissive of infrared radiation, an opaque mask secured to said envelope and disposed between said window in said envelope and said frame so as to overlay a portion of said layer of semiconductor to prevent radiation passing through said window from impinging on said overlayed portion of said layer.

2. A bolometer comprising an evacuated envelope, a window in said envelope formed of a material transmissive of infrared radiation, an annular glass frame fixedly positioned in said envelope opposite said window J and having a central opening therethrough, a thin film of insulating material secured to one side of said frame to cover the opening thereof, a thin elongated layer of semiconductor material having a negative thermo-coeflicient of resistance on said film, and terminals connected to spaced-apart portions of said layer.

3. A bolometer comprising an evacuated envelope, a window in said envelope formed of a material transmissive of infrared radiation, an annular glass frame fixedly positioned in said envelope opposite said window, a thin film of insulating material secured to one side of said frame to cover the opening thereof, three parallel spaced strips of metallic film on said insulating film, the middle one of said three strips extending diametrally across said frame and the opening thereof, a thin elongated layer of semiconductor material on said insulating film extending across said strips, said layer conductively contacting all three of said strips, the opposte ends of said layer terminating on the outer strips respectively, and three terminals conductively connected to said three strips, respectively.

4. A bolorneter comprising an evacuated envelope, a window in said envelope formed of a material transmissive of infrared radiation, an annular glass frame fixedly positioned in said envelope opposite said window, a thin film of insulating material secured to one side of said frame to cover the opening thereof, three parallel spaced strips of metallic film on said insulating film, the middle one of said three strips extending diametrally across said frame and the opening thereof, the two outer strips extending along chords of said frame adjacent to the periphery of the frame opening whereby the film of insulating material between said strips is exposed and free of substantially all conductive material with the exception of the middle strip, a thin elongated layer of semiconductor material on said insulating film extending across said strips, said layer conductively contacting all three of said strips, the opposite ends of said layer terminating on the outer strips respectively, and three electrode terminals passing through companion apertures in said frame in registry with said three strips respectively, said three terminals being conductively connected to the respective strips.

5. The bolometer of claim 4 wherein the layer of semiconductor material extends at right angles with respect to said strips.

6. The bolometer of claim 4 including an opaque mask secured to said envelope and positioned between said window and said layer, said mask overlying one-half of the length of said layer extending from the middle strip to one outer strip to prevent radiation passing through said window from impinging said one-half of said layer.

References Cited in the file of this patent UNITED STATES PATENTS 2,096,170 Geisler et al. Oct. 19, 1937 2,163,393 Brunke et al. June 20, 1939 2,248,614 Ferrant July 8, 1941 2,448,516 Cashman Sept. 7, 1948 2,493,745 Blodgett et a1. Jan. 10, 1950 2,556,991 Teal June 12, 1951 2,651,009 Meyer Sept. 1, 1953 2,669,663 Pankove Feb. 16, 1954 2,892,250 Bartels June 30, 1959 2,953,690 Lawson et al Sept. 20, 1960 OTHER REFERENCES J. Opt. Soc. Am., vol. 45, page 27, January 1955, article by L. Harris. 

